1. Field of the Invention
The present invention relates to amplifiers using shunt-feedback, for example low noise amplifiers having shunt-feedback.
2. Background Art
Various RF input circuits include front-ends that require a large variable gain range. In all applications where a large variable gain range is required, such as TV front-ends, low noise amplifiers (LNAs) typically do not use feedback. This leads to lower dynamic range, higher noise, lower intercept point and higher power consumption.
The use of resistive shunt feedback can improve the dynamic range (which is proportional to the ratio of the intercept point and the noise figure) of an impedance matched low noise amplifier. However, the use of feedback in an impedance-matched variable gain amplifier is complicated because, as the gain varies, so does the input impedance, limiting the variable gain range to a few dBs (typically less than 6 dB). In fact the input impedance is given by the feedback resistance divided by 1 plus the amplifier gain (Rf/(1+A)).
Therefore, what is needed is an impedance-matched variable gain amplifier that includes a feedback mechanism to improve the dynamic range, but that does not adversely effect the input impedance.